David Cameron made an unscheduled statement in the House of Commons on the UN decision to authorise military action against Gaddafi's regime. Photograph: Ho/Reuters

Quiet parliamentary Fridays can prove to be ominous days for the House of Commons. On a Friday in 1939 Germany invaded Poland. On a Friday in 1983 the Argentine junta sought to save itself from the wrath of its own people by invading the Falkland Islands.

Today David Cameron became the latest prime minister to make an unscheduled statement as war clouds loom.

A few minutes ago he spoke – quietly and well – about the implications for Britain's hard-pressed military and taxpayers of the UN security council's decision to authorise military action against the rogue regime in Libya.

In August 1914 there was jubilation in European capitals at the prospect of quick victory, a delusion on all sides that was not repeated in 1939 – or since. Even the Sun's "Gotcha" and Margaret Thatcher's "Rejoice, rejoice" triumphalism in 1983 now seems embarrassingly misplaced.

Today's mood was sombre, as it should be. We can't afford this extra expense, but we can't afford not to afford it either. No, we can't intervene on humanitarian grounds everywhere, Cameron conceded to the leftwinger Jeremy Corbyn. It doesn't mean we can't do what we can, especially when an unstable "pariah state" lingers on Europe's southern flank. I don't think he mentioned oil and gas.

Flanked by Nick Clegg and William Hague, both looking suitably grave, Cameron laid out the three tests he deems necessary to justify the "exceptional circumstances" that warrant outside interference in a state's internal affairs.

"Demonstrable need" had been demonstrated by 1,000 deaths already and the prospect of a front assault on Benghazi, a 2,500-year-old city of one million people, said Cameron. "Regional support" had been shown by Arab League willingness to join military action and by the votes of all three African members of the UN security council.

So far so good. Cameron finds himself sounding more like Tony Blair than he must have intended a year ago, but he seeks to underpin the coming gamble – one in which he has been a zealous activist – by making distinctions. So point No 3, "a clear legal basis", was also underlined.

A UNSC resolution, the backing of appropriate authorities and the blessing of the attorney general, also visible today on the government bench – they were all there, along with emphasis on limited action. Cameron rounded off by urging Muammar Gaddafi to cooperate – shades of Saddam Hussein in 2003? – and a pledge to "do right by the Libyan people".

By chance the PM's statement interrupted a self-indulgent debate among Tory Euro-sceptics about the need to reassert British sovereignty against the European Union, as if the Iraq war had not demonstrated that member states can assert their sovereignty as long as they retain the political will and military capacity.

It did not stop that old warhorse, Bill Cash MP, greeting Cameron's arrival as proof that Britain could act – despite EU "impediments". I think he means the Germans, whose reluctance to go to war is a habit we spent much of the 20th century encouraging. Bill Cash's father was one of those who paid the price.

In the real world Cameron's actions reflect a more layered, nuanced sovereignty that is part-national, part pooled and dependent on consensus and will. After France initially miscalled the Arab revolt – failing to back Tunisia or Egypt in rebellion – Nicolas Sarkozy has been running hard to catch up. Paris will host a meeting with Arab leaders on Saturday.

Ed Miliband offered Labour support – as the Labour opposition did on those earlier Fridays – and asked more detailed questions about diplomatic activity, about humanitarian aid and other issues – as opposition leaders should. Cameron turned up early – good – and there will be a full Commons debate on Monday – good again. Dissent needs to be heard.

But not much of it today. Sir Malcolm Rifkind, a cheerleader for intervention as he was not in Bosnia, when he was foreign secretary in the mid-90s, praised Cameron's leadership. Don't forget the French, added Labour's Mike Gapes, another foreign affairs specialist, who wondered what role the African Union will play.

Notably absent in all this was any serious discussion of precisely what military hardware Britain can or will bring to bear against the Gaddafi regime. Cameron made a breezy passing reference to Tornado and Typhoon aircraft, and other force multipliers – surveillance aircraft, warships – that can be deployed, all short (he kept stressing) of "boots on the ground". The DUP hardman, Jeffrey Donaldson, urged No 10 to reconsider the special strategic defence review (SSDR) which has just further slashed British military capacity. It was such a money-saving defence cut in 1982-83 that gave the junta its green light – an expensive mistake that Margaret Thatcher smothered by dispatching a 40,000-strong task force. We could not do it now.

Cameron insists that, even with the latest cuts, Britain retains the fourth largest defence budget and thus – did he say this explicitly? – the fourth largest defence force, one that is increasingly flexible in deployment.

But the silence on detail reflects two unavoidable facts. One is that – as usual – the US will do and pay for most of the action now that its initial reticence, tactical as well as domestic, has been abandoned. Africans, Arabs, Europeans, all may take part – Cameron was vague – but American aircraft, predator drones and warships will decide the matter with the maximum display of resolve (a spot of "shock and awe"?) deemed prudent.

The second reason is that few MPs nowadays have any military experience – not the case a generation ago, when world war two vets were still around. That situation is actually improving – ironically – because of western interventionism. Major Dan Jarvis of 1st Para was elected Labour member for Barnsley Central only this month.

Today it fell to Nick Soames, ex-soldier, ex-defence minister and Churchill scion, to urge greater EU military cooperation on Cameron (not a popular Tory line, but realistic) and to Colonel Bob Stewart, of Balkans fame, to ask exactly what "all military means" actually means. He spoke as a man who had seen what Bosnian Serb militia could do to civilians.

Cameron was polite, articulate and blandly reassuring to all comers. His energies will be drawn into this conflict now – days before George Osborne's budget – and there are only 24 hours in a day.

The global financial crisis is not over, the EU and the US are at odds over economic strategy (Britain's coalition is in the EU austerity camp!), and the wave of disasters sweeping over Japan remain unresolved. Stability in North Africa would be a useful contribution – if that is how things now turn out rather than another lurch towards the abyss.

But the world's agenda is dangerously over-burdened. We could all do without this. Fingers crossed.